Generally, an internal combustion engine is a heat engine for reciprocating a piston by explosively burning a fuel mixed with air in cylinders. Exhaust gases generated in burning are exhausted externally through an exhaust apparatus 10, as shown in FIG. 1, comprising an exhaust manifold 12 collecting the exhaust gases in each of the cylinders, an exhaust pipe 14 for exhausting them into the exterior, a muffler 16 for reducing an exhaust noise, and a catalytic converter 18 for oxidizing and reducing noxious components in the exhaust gases to thereby make them harmless. However, since harmless components such as unburned hydrocarbon, carbon monoxide, nitrogen oxide, sulfur oxide, etc. are contained in the exhaust gases, the exhaust gases exhausted from the cylinders should be collected, purified at a purification system disposed at a middle of the exhaust pipe 14, and then exhausted to the exterior.
A purifier using 3-way catalyst, low temperature plasma, a combination of the 3-way catalyst and the low temperature plasma, and a photocatalyst, etc is used as a purification system.
The purification system for using the 3-way catalyst utilizes precious metals capable of catalyzing, that is, platinum(Pt)+rhodium(Rh) or platinum+rhodium+palladium(Pd), to thereby simultaneously reduce carbon monoxide, hydrocarbon, nitrogen oxide in the exhaust gases and, in high temperature, to have an excellent purification effect of 98% or more.(see SAE982606). Therefore, in recent, the purification for using the 3-way catalyst is frequently used.
However, in the case of purifying the exhaust gases by using the 3-way catalyst, it has a shortcoming in that heat is needed to activate the catalyst as well as the catalysis thereof is performed only at a predetermined temperature. That is, before the catalyst is activated at the predetermined temperature, such as an initial stage of starting an engine for vehicles, noxious components are not smoothly removed. In other words, when the catalyst has not reached a specific active temperature, the exhaust gases are exhausted in air just as the hydrocarbon that is not purified.
Further, in order to perform both reactions of oxidation and a reduction, since it must be close to a theoretic mixture ratio, it has a shortcoming in that an exhaust condition is restricted. Accordingly, when only the mixture ratio is close to the theoretic mixture ratio the noxious components such as unburned hydrocarbon, carbon monoxide, and nitrogen oxide, etc. are reduced. In other words, when a fuel is rich, the purification to hydrocarbon and carbon monoxide is suddenly reduced, while when air in the fuel is rich, the purification to nitrogen oxide is suddenly reduced.
Recently, it has been studied in various fields in order to improve a fuel rate and to reduce a deflection of carbon dioxide for reducing a green house effect.
For example, techniques regarding a lean burn engine or a gasoline direct injection engine(GDI) have been proposed, but since a large amount of oxygen exists in the exhaust gases, it is a shortcoming in that a 3-way catalyst cannot be used.
That is, in case of the lean burn engine or the gasoline direct injection engine, since the engines are driven at a rich supply of air, an oxygen of 10% and more and a large amount of nitrogen oxide exists in the exhaust gases depending upon a lean burn combustion condition. Thus, it is a restriction that the large amount of nitrogen oxide cannot be sufficiently purified by only the 3-way catalyst.
Specifically, in case of a diesel engine, it is a problem that a particulate material is generated using a low grade fuel, a large amount of nitrogen oxide is produced by the lean burn and the purification capability of the exhaust gases is remarkably deteriorated by oxygen.
In order to overcome these problems, a nitrogen oxide reducing system and a nitrogen oxide absorbing system using low temperature plasma are recently used. These purification systems are mainly used as a fixed internal combustion engine or a desulphurization or denitration system of a large engine to thereby purify nitrogen dioxide in the exhaust gases by using a reducing agent such as urea or ammonia, etc. into nitrogen and oxygen.
These low temperature plasma purification systems comprise electrodes in an induction tube in which exhaust gases flow, the electrodes being supplied with a power supply such as the direct current (DC) or an alternate current (AC) in order to generate the plasma. When the exhaust gases pass through the induction tube, moisture, oxygen or nitrogen and the like existing in the exhaust gases are ionized or dissociated by the low temperature plasma to thereby generate a free radical, thereby purifying contaminants. (See SAE982428)
However, since these low temperature plasma purification systems need a high energy and a supplying apparatus and since a reactor is relatively bulky relative to an amount of exhaust gases, a matter to be purified is limited to nitrogen oxide and sulfur oxide. That is, even though these systems are suitable to a fixed internal combustion engine for reducing hydrocarbon and nitrogen oxide of a low concentration of about 1000 ppm, it is a shortcoming in that enormous energy corresponding to 2% of an internal combustion engine output is consumed in order to activate the plasma as well as a volume of the respective systems is increased 10 times or more. Further, since the systems are bulky in a large installation space, it is unsuitable to be used with general automotive vehicles requiring a moving activity and restricting a useful energy.
In order to reduce noxious components in the initial stage of starting at cold temperature, energy is supplied from a power supply of a condenser, not from a generator. Thus, since energy capacity is small in an energy system of the existing vehicles, the purification of the exhaust gases cannot be performed, while when energy capacity is increased, it should be concomitant with subsidiary facilities, causing a cost as well as an installation problem. Further, in order to reduce relatively high unburned hydrocarbon components of about 6000 ppmC in the exhaust gases, a plasma reactor having significant large volumes and a predetermined space needs to install the plasma reactor in vehicles, but, since the installation space of vehicles is limited as is generally known, it is unrealistic to install a high volumetric plasma reactor in vehicles.
Furthermore, an additive such as urea and unburned hydrocarbon needs to convert nitrogen oxide under an oxidation atmosphere. The additive is easily supplied in a fixed type internal combustion engine, but in case of vehicles, it is a problem that an additive supplying system is additionally mounted in the vehicles and it is difficult to secure an installation space of the supplying system in the vehicles and it is hard for drivers to get to continuously supply the additive at a regular interval such as at a time of a fuel filling.
Recently, a system of combined low temperature plasma purification system with the 3-way catalyst purification system has been investigated. That is, the 3-way catalyst purification system is disposed to a backward portion of a plasma reactor to thereby purify unburned hydrocarbon untreated by plasma reaction (See SAE982427, 982429, 982508).
However, since the combination system consumes high energy for generating the plasma and the volume thereof is bulky, it is not preferable to use in a moving type internal combustion engine.
On the other hand, a purification system using a photocatalyst irradiates a photic source having a specific wavelength to the photocatalyst, for example TiO2, and then purifies contaminants by a free radical generated in exciting the photocatalyst. Further, the photocatalyst takes part in a purification reaction of nitrogen oxide as well as an oxidation reaction of carbon monoxide and hydrocarbon, thereby performing an activation without regard to energy or temperature condition (J. of Photochemistry and Photobiology AL Chemistry 111, pp199–203, 1997).
The purification system may use a wavelength contained in a natural light as a photic source, but the photic source needs a specific wavelength in order to active the photocatalyst, thererby increasing an effect. For example, Japanese Laid-open patent Nos. 1994-10652 and 1998-169431 disclose an exhaust gas purification system using a corona discharge and a 3-way catalyst and using an integrally formed plasma generating system with a NOx catalyst system, respectively. As disclosed in these patents, these systems need use of an ultraviolet lamp generating a wavelength of 200–400 nm, but the ultraviolet lamp can convert only 20% of an input energy to an optical energy and convert the remaining energy thereof to a heat energy, resulting in that an energy effect is extremely low, the lifecycle thereof is short and the maintenance cost is high.
On the other hand, it has been proposed a purification method capable of purifying contaminants already exhausted in air by an oxidation method using a bio-filter, an active carbon and an ultraviolet.
The purification method using a bio-filter can biochemically dissolve an organic or non-organic atmospheric contaminant, the method comprising of the steps: placing biochemical active materials to a carrier such as a soil and forcibly circulating air in the carrier, while that using an active carbon comprising of the steps: storing contaminants in carbon for a short time and treating the stored contaminants in a lump. Further, the purification method using an ultraviolet can oxide hydrocarbon by using a sterilization due to an ozone generated when an ultraviolet is irradiated and a radical of oxygen ion and hydrogen ion generated by dissolving water and, for example, the purification method is disclosed to Japanese Laid-open patent Nos. 1999-091345, 1998-244129 and 1998-192654.
However, the above patents employing the above described purification method are a fixed type purification system which is designed to be fixed in place to have a specific amount. Accordingly, although the patents may be useful for purifying an indoor air of a large sized building, e.g., a limited amount of air, they are still inadequate to freely stick to a purification amount because of extra installation expense and an operating cost are required therefore.